Decimal reduction time and inactivation rate of Vibrio vulnificus and Vibrio parahaemolyticus in oyster meat by cryogenic freezing (CF) and air-blast freezing (AB) and their impact on oyster meat quality and microstructure

Raw oyster meats may be contaminated with Vibrio parahaemolyticus and V. vulnificus. These Vibrio species have been strongly associated with severe and life-threatening conditions in immunocompromised individuals. Post-harvest treatment, such as freezing, affects Vibrio species present in oyster meats and the quality of oysters. This study (a) determined the effect of freezing rate associated with cryogenic freezing (CF) and air-blast freezing (AB) in the removal of heat load and their effect on oyster meat quality, and (b) determined the decimal reduction time and k (inactivation rate) values of Vibrio vulnificus and Vibrio parahaemolyticus in oyster meats that had undergone either CF or blast-freezing (AB) followed by 60 days of storage at −20 °C±1 °C. The freezing times for AB and CF oyster meats were 27.9 ± 0.9 and 1.7 ± 0.5 min, respectively. CF oyster meat had a higher freezing rate and energy removal rate (9.9 ± 0.4 °C/min and 23.7 ± 0.8 J/s, respectively) than AB samples (0.6 ± 0.1 °C/min and 1.3 ± 0.2 J/s, respectively). Microscopic observation of oyster meat tissue showed that the lower freezing rate in AB caused severe deformation of muscle tissue. CF and AB had similar D values in reducing V. vulnificus and V. parahaemolyticus in oyster meats. CF and AB reduced V. vulnificus to nondetectable levels by 30 days of storage, while V. parahaemolyticus was decreased to nondetectable levels within 60 days in CF and AB samples. The TBARS (thiobarbituric-acid-reactive-substances) value for CF oyster meat after six months was 47.7% lower than AB, which demonstrated that a higher freezing rate in CF before storage triggered less lipid oxidation compared to AB during the storage. This study showed that freezing could reduce Vibrio spp. to nondetectable levels during storage, and the freezing rate could influence the quality of oyster meats.